Novel scr catalysts and after-treatment devices for diesel engine exhaust gas

ABSTRACT

Disclosed is an SCR catalyst composition having improved NOx conversion and a device for post-treating diesel engine exhaust gas using the same. The SCR catalyst composition can effectively convert NOx generated from an engine into harmless components. The device, which improves the NOx conversion, includes an SCR catalyst module disposed downstream of a diesel engine and having an injector, an oxidation catalyst module disposed downstream of the SCR catalyst module, and a catalyst soot filter module, all of which are successively arranged in series.

TECHNICAL FIELD

The present invention relates to an SCR (Selective Catalytic Reduction) catalyst composition having improved NO_(x) conversion and a device for post-treating diesel engine exhaust gas using the same. More particularly, the present invention relates to an SCR catalyst composition which is capable of effectively converting NO_(x) in diesel engine exhaust gas into harmless components, and to a device for emitting exhaust gas, which has an improved capability to purify diesel engine exhaust gas, in particular, NO_(x), using the catalyst composition.

BACKGROUND ART

Generally, a diesel engine having inherently high heat efficiency (i.e. high fuel economy) and high torque at low speed is useful as a device for providing driving force to a vehicle, and may be driven at a high A/F (air to fuel) ratio under conditions of dilute fuel mixture. For this reason, the diesel engine exhibits high HC and CO conversions, but has high NO_(x) and particulate emissions. Both NO_(x) and particulates are diesel exhaust gas components difficult to convert into harmless components, and the emission standards related thereto are increasingly restricted because of associated environmental problems. In particular, various methods for removal of NO_(x) are devised. Further, an SCR catalyst system or a NO_(x) adsorption (lean NO_(x) trap) catalyst system may be proposed, but the present invention is directed to SCR techniques.

FIG. 1 schematically shows an SCR system. The SCR reduces NO_(x) into N₂ and H₂O using ammonia or urea as a reducing agent as below.

4NO+4NH₃+O₂

4N₂+6H₂O

2NO₂+4NH₃+O₂

3N₂+6H₂O

NO+NO₂+2NH₃

2N₂+3H₂O

In the SCR system, the SCR catalyst is exemplified by a V₂O₅/TiO₂ catalyst. The V₂O₅/TiO₂ catalyst is operated in the temperature range of 250˜450° C. However, particularly in the case where exhaust gas contains only NO, low-temperature performance of the catalyst is not ensured. On the other hand, in the case where the exhaust gas contains a large amount of NO₂, the catalyst has improved performance and a wide low-temperature operating range. Thus, an oxidation catalyst is disposed upstream of the SCR system such that NO is oxidized into NO₂ to increase the concentration of NO₂. As such, if the NO₂/NO ratio is too high, the catalyst performance becomes somewhat deteriorated, and thus, NO₂/NO=1 is known to be optimal. In this regard, a typical patent (hereinafter, referred to as the “JMI patent”) refers to a NO_(x) reduction system in exhaust gas (PCT/GB/00292, WO 99/29809). FIG. 2 schematically shows the system of the reference patent. This system is configured such that an oxidation catalyst for converting NO of NO_(x) generated from the engine into NO₂, a particulate filter, a reducing agent injector, and an SCR catalyst are sequentially disposed. The SCR system thus configured is disclosed to exhibit NO_(x) conversion of 80% or more. In the JMI patent, a V₂O₅/WO₃/TiO₂ SCR catalyst is employed.

In the JMI patent, the particulate filter for decreasing a particulate concentration is a catalyzed diesel particulate filter (CDPF), called a catalyzed soot filter (CSF). As a brief description, this filter is configured such that particulates in exhaust gas are trapped and the filter is regenerated, and is exemplified by various known filter structures usable to trap particulates in diesel exhaust, including a honeycomb wall-flow filter for trapping particulates. The particulates trapped in the filter and accumulated thereon should be continuously or periodically combusted from the filter (for filter regeneration). Accordingly, to lower the combustion temperature adapted therefor, a catalyst is provided, thus forming the CDPF. The CDPF or CSF may remove 90% or more of particulates, along with the regeneration of the filter following the passive combustion of particulates accumulated thereon.

DISCLOSURE Technical Problem

Therefore, the present inventors have directed their attention to the problems of the SCR catalyst mentioned in the JMI patent in that 1) performance of an oxidation catalyst disposed upstream of the SCR catalyst should be concurrently considered because NO_(x) conversion depends on the NO₂/NO ratio and 2) NO_(x) conversion is not efficient at low temperatures attributable to poor low-temperature activity of the SCR catalyst. To solve these problems, the SCR catalyst is required to be newly designed. The present inventors have developed a novel SCR catalyst based on various catalyst design methods, and the SCR catalyst according to the present invention has been confirmed to exhibit superior low-temperature activity and thus effectively react even at the temperature of engine exhaust gas and as well achieve desired NO_(x) conversion even in the absence of an oxidation catalyst. Hence, when the SCR catalyst according to the present invention is employed, it is possible to realize catalyst configuration very different from that in the conventional JMI patent, and thus, a device exhibiting high NO_(x) conversion and for emitting diesel engine exhaust gas may be provided, with no consideration of a relation between the conventional SCR catalyst and the oxidation catalyst disposed upstream thereof. FIG. 3 schematically shows the device for emitting diesel engine exhaust gas according to the present invention.

Technical Solution

An object of the present invention is to provide an SCR catalyst composition having superior low-temperature activity and high NO_(x) conversion. Thanks to the high NO_(x) conversion of the SCR catalyst according to the present invention, use of an oxidation catalyst which should essentially be disposed upstream of the conventional SCR catalyst may be excluded.

Another object of the present invention is to provide a device for emitting diesel engine exhaust gas which uses the SCR catalyst having superior low-temperature activity and high NO_(x) conversion.

In order to accomplish the above objects, the present invention provides a device for emitting diesel engine exhaust gas, composed of an SCR catalyst module disposed downstream of a diesel engine and including an injector, an oxidation catalyst module disposed downstream of the SCR catalyst module, and a CSF module, all of which are successively arranged in series.

The SCR catalyst composition according to the present invention includes Cu or Fe-containing zeolite impregnated with one or a combination of two selected from the group consisting of Pt, Pd and Rh; or Cu, Fe or Si-containing alumina impregnated with one or a combination of two selected from the group consisting of Pt, Pd and Rh.

Advantageous Effects

According to the present invention, the SCR catalyst can exhibit high NO_(x) conversion at low temperatures, regardless of the NO₂/NO_(x) ratio which is optionally adjusted by a conventional diesel oxidation catalyst. The use of such a SCR catalyst enables the flexible design of a diesel engine exhaust system.

DESCRIPTION OF DRAWINGS

FIG. 1 schematically shows an SCR system;

FIG. 2 schematically shows a diesel engine exhaust system in a conventional JMI patent;

FIG. 3 shows a diesel engine exhaust system according to the present invention; and

FIG. 4 shows a graph of NO_(x) conversions of the comparison catalysts and the novel catalyst composition according to the present invention.

MODE FOR INVENTION

According to the present invention, the exhaust system is configured such that an SCR catalyst module, a diesel oxidation catalyst (DOC) module and a CSF module are successively arranged in series, as shown in FIG. 3.

In the present specification, the exhaust gas ‘downstream’ indicates the direction of exhaust gas flow from an engine. The ‘module’ indicates a device of a minimal unit for performing a single function. For example, the ‘CDPF module’ indicates a filter device for trapping particulates emitted from a diesel engine, and the ‘DOC module’ indicates a carrier coated with so-called DOC for oxidizing HC or CO emitted from the diesel engine into harmless components.

According to an embodiment of the present invention, a device for emitting diesel engine exhaust gas includes an SCR catalyst module for receiving exhaust gas emitted from a diesel engine to convert NO_(x) in the exhaust gas using a reducing agent which is introduced through a lateral side of the module, a DOC module disposed downstream of the SCR catalyst module to oxidize HC and CO, and a CSF module for trapping particulates, all of which are successively arranged in series.

Below, respective modules are described in detail.

1) SCR Catalyst Module

The SCR catalyst improved according to the present invention includes Cu or Fe-containing zeolite impregnated with one or a combination of two selected from the group consisting of Pt, Pd and Rh; or Cu, Fe or Si-containing alumina impregnated with one or a combination of two selected from the group consisting of Pt, Pd and Rh. Such a catalyst is applied on a carrier made of ceramic such as cordierite or fireproof metal such as stainless steel, thus forming the SCR catalyst module. A typical example of the carrier includes a flow-through type carrier having a plurality of fine channels extending between opposite sides thereof and thus communicating the opposite sides with each other. The SCR catalyst module includes a known injector disposed upstream thereof to spray a reducing agent. For example, the injector may include a pump and an injection nozzle for spraying urea or ammonia acting as a reducing agent.

A method of preparing an SCR catalyst composition useful for the SCR catalyst module according to the present invention is described below. The SCR catalyst composition used in the present invention may be prepared by impregnating 5-10 wt % Cu-containing zeolite with 0.5 wt % or less Pt, thus preparing a slurry, coating a cordierite- and metal-made carrier with the slurry, and then performing drying and burning. Alternatively, using the same method as above, 5˜10 wt % Fe-containing zeolite may be impregnated with Pt in the same amount by weight. Instead of the zeolite, alumina may be used as a support. As such, the alumina may be alumina containing Cu, Fe or Si.

To evaluate the NO_(x) conversion performance of the SCR catalyst according to the present invention, a conventional SCR catalyst (V₂O₅/TiO₂) alone, a combination of the conventional SCR catalyst and a DOC catalyst (Pt/Al₂O₃), and the novel SCR catalyst according to the present invention were measured for NO_(x) conversion under conditions of NH₃/NO_(x)=1. The results are shown in FIG. 4. As is apparent from these results, the SCR catalyst according to the present invention could be seen to very effectively convert NO_(x) at 250˜280° C. corresponding to the actual temperature of diesel engine exhaust gas.

Specifically, as results of application of the SCR catalyst according to the present invention in the temperature range of 250˜280° C. corresponding to the temperature of diesel engine exhaust gas, the NO_(x) conversion of about 90% or more was obtained, which is improved compared to NO_(x) conversion of 60˜80% upon use of the conventional SCR catalyst alone. When compared with the conversion of about 80% occurring as a result of the combination of the SCR catalyst and the DOC which is essentially disposed upstream thereof to optimally set the NO₂/NO ratio in the JMI patent, the novel SCR catalyst according to the present invention can be confirmed to considerably improve the conversion.

2) DOC Module

The DOC oxidizes HC and CO in diesel engine exhaust gas and thus converts them into harmless components, removes 50˜80% of a soluble organic fraction of particulate matter (PM), and lessens diesel odor and black smoke. As a catalyst, Pt or Pd is used, and for example, a Pt/AlSO₃ catalyst is known as a typical DOC catalyst. As the DOC catalyst used in the present invention, a known module may be employed.

3) CDPF or CSF Module

A typical diesel particulate module is a filter for trapping PM emitted from a diesel engine, and includes for example, various known filter structures, such as a ceramic monolith filter, a ceramic fiber filter, and a sintered metal filter. Particularly useful is a ceramic monolith filter, called a ceramic wall-flow filter composed of porous ceramic such as cordierite. This filter has a plurality of channels in which each of the channels is closed at one end thereof such that opposite inlet and outlet ends of the channels are alternately closed. The exhaust gas introduced into one channel through an inlet end thereof passes through a porous wall between the channel and the adjacent channel and enters the adjacent channel, and then the exhaust gas is discharged to the outside through the outlet end of the adjacent channel, because the one channel is closed at the outlet end opposite the inlet end. As a result of the above flow of exhaust gas, the PM is trapped in the channels. Because the pore size of the porous wall of the filter has a direct influence on a diameter of trapped PM and engine back pressure, it is regarded as important for selecting the type of module. Preferably, selected is a module in which the pore size is set to about 10 μm, and in such a porous structure, PM having a diameter of 0.01 μm or more may be trapped, and 85% or more of PM is removed. The PM trapped in the filter is ignited within as short a time as possible and the filter is regenerated to a state in which it can trap PM again. Further, to lower the trapped PM ignition temperature, 550˜600° C., to about 250° C., depending on the manner of applying the catalyst to the channels of the filter, a CRT (Continuous Regeneration Trap) and DPX module may be employed. The usable module is exemplified by CSF 12 composed of 250 g/ft³ ZrO₂, 500 g/ft³ CeO₂, 75 g/ft³ Pt, available from ENGELHARD. As the DOC catalyst used in the present invention, a known module may be employed. 

1. A device for emitting diesel engine exhaust gas, suitable for improving NO_(x) conversion, comprising an SCR catalyst module disposed downstream of a diesel engine and including an injector, an oxidation catalyst module disposed downstream of the SCR catalyst module, and a catalyst soot filter module, all of which are successively arranged in series.
 2. The device according to claim 1, wherein an SCR catalyst used in the SCR catalyst module comprises Cu or Fe-containing zeolite impregnated with one or a combination of two selected from the group consisting of Pt, Pd and Rh; or Cu, Fe or Si-containing alumina impregnated with one or a combination of two selected from the group consisting of Pt, Pd and Rh.
 3. The device according to claim 2, wherein the catalyst is applied on a carrier made of a ceramic or a fireproof metal including stainless steel.
 4. The device according to claim 2, wherein the SCR catalyst comprises 5˜10 wt % Cu or Fe-containing zeolite impregnated with 0.5 wt % or less of one or more selected from the group consisting of Pt, Pd and Rh.
 5. The device according to claim 2, wherein the SCR catalyst comprises 5˜10 wt % Cu, Fe or Si-containing alumina impregnated with 0.5 wt % or less of one or more selected from the group consisting of Pt, Pd and Rh. 